In an electric power steering apparatus, mechanical relays are generally mounted on a power supply line and a motor line for the purpose of failsafe. Hermetically sealed relays are widely used as these relays in order to prevent the ingress of moisture, which causes the freezing of contacts during time in a low-temperature environment, as much as possible.
Further, since the relays have been also further miniaturized by demands on the miniaturization with high output of a system, the internal volume of the relays tends to be reduced. Meanwhile, a resin is generally used as a material of a case used for the relay. For this reason, if the relay is left under a high-temperature and high-humidity environment for a long time, the resin absorbs moisture in the air even though airtightness of the relay is ensured. Accordingly, relative humidity in the relay significantly increases particularly in the relay having a small internal volume. As a result, dew condensation is likely to occur on relay contacts.
In addition, a method of directly mounting a relay on a metal substrate and soldering the relay by use of a reflow oven is employed for the purpose of the improvement of heat dissipation. Since the cooling rate of the relay is high when the relay is mounted on a metal substrate as described above, moisture present in the relay is frozen under the low-temperature environment. Therefore, contact failure of relay contacts caused by the freezing of the contacts in likely to occur.
Accordingly, a technique for preventing the contact failure of the relay contacts is disclosed in, for example, PTL 1. In this technique, a foreign substance such as ice adhering to the relay contacts is removed by the repetition of an operation of turning on/off the relay. In addition, another technique disclosed in, for example, PTL 2. This technique is to generate vibration at a portion where freezing occurs and to release the freezing by this vibration.